The following documents, each published in the name of Dallas B. Noyes, disclose background information hereto, and each is hereby incorporated herein in its entirety by this reference:    U.S. Patent Publication No. 2012/0034150 A1, published Feb. 9, 2012;    International Application No. PCT/US2013/000071, filed Mar. 15, 2013;    International Application No. PCT/US2013/000072, filed Mar. 15, 2013;    International Application No. PCT/US2013/000073, filed Mar. 15, 2013;    International Application No. PCT/US2013/000075, filed Mar. 15, 2013;    International Application No. PCT/US2013/000076, filed Mar. 15, 2013;    International Application No. PCT/US2013/000077, filed Mar. 15, 2013;    International Application No. PCT/US2013/000078, filed Mar. 15, 2013;    International Application No. PCT/US2013/000079, filed Mar. 15, 2013; and    International Application No. PCT/US2013/000081, filed Mar. 15, 2013.
The energy released by radioactive decay is much greater than the energy released by chemical reactions. Nuclear batteries take advantage of the energy density of radioisotopes and overcome some of the deficiencies of conventional electrochemical batteries (e.g., alkaline batteries, nickel-metal hydride batteries, etc.), such as limited energy content, sensitivity to environmental conditions, and relatively short useful life. Nuclear batteries typically have higher unit costs than conventional electrochemical batteries and have greater safety concerns.
Nuclear batteries are either indirect or direct energy conversion devices. In indirect conversion devices, radiation energy is converted to light or heat, which is then converted to electricity (e.g., by a photovoltaic cell or a thermopile). In direct conversion devices, radiation energy is converted directly to electricity without any intervening conversions.
In a direct-conversion nuclear battery, a radiation source emits radiation that is received by a doped semiconductor material having a p-n junction. The radiation (typically beta particles) causes a change in the electric potential of the doped semiconductor material, which can provide an electric potential to a load electrically connected to the doped semiconductor material. Radiation penetrates one semiconductor material and passes the junction into the other semiconductor material.
Some nuclear batteries include Schottky barriers. Schottky barriers include a semiconductor layer coated with a metallic layer, the junction between the layers having rectifying characteristics. For example, Schottky barriers are described in U.S. Pat. No. 5,859,484, issued Jan. 12, 1999, and titled “Radioisotope-Powered Semiconductor Battery;” in F. K. Manasse et al., “Schottky Barrier Betavoltaic Battery,” IEEE Transactions on Nuclear Science, Vol. NS-23, No. 1, pp. 860-70 (February 1976); and in Jasprit Singh, Semiconductor devices: Basic Principles, 221-244 (Wiley 2001). In a nuclear battery having a Schottky barrier, current flows when the Schottky barrier is excited by radiation. Schottky barriers directly convert the energy of decay particles into electricity.
One of the key issues in direct-conversion nuclear batteries is the fraction of the decay particles striking the conversion device. The efficiency of direct-conversion devices tends to be limited because the decay products disperse in all directions. Because decay particles can be emitted in any direction, the fraction of the decay particles striking the conversion device depends on the geometry of the conversion device and the location of the radioactive source. For example, the probability of any particular decay particle's striking a planar surface near the radioactive element is less than 50%. The term “capture efficiency” of a conversion device is used herein to describe the fraction of decay particles leaving a radiation source that interacts with that conversion device. Energy of particles leaving a radiation source that do not interact with the conversion device is neither collected nor converted.
Conversion devices have been developed with various geometries to increase the capture efficiency. Such geometries include contoured surfaces, and channels, holes, grooves, and corrugations in the surfaces of the conversion device. U.S. Pat. No. 5,396,141, issued Mar. 7, 1995, and titled “Radioisotope Power Cells,” discloses a radiation source sandwiched between two semiconductor materials or contained within a trench defined by the semiconductor materials. The trench is configured to have an aspect ratio of approximately 20:1 to increase the likelihood of any particular radioactive particle impinging on the semiconductor material. It would be advantageous to provide a nuclear battery having a conversion device with a higher capture efficiency and therefore a higher overall conversion efficiency than is currently available.